Several attempts have been made in the prior art to control the motion of a film in moving picture projectors by means of a light beam. The principle of these inventions of the prior art has been to convert the speed of the film into an electric current by passing the film apertures between a light source and a light sensitive element and use the fluctuating electric current created in the light sensitive element to drive a variable speed electric motor which adjusts the speed of an optical compensator with the speed of the film.
However, to be able to pass the film between the light source and the light sensitive element one has to be placed inside and the other outside a hollow roller. Consequently one side of the roller has had to be left open and the various shafts of the roller had to protrude from the other side, concentrically which caused difficulties in the construction and prevented the design of a simple and easily installable device.
Another disadvantage of the system of the prior art was because the electric motor had to adjust continuously the speed of the film in relation to the speed of the compensator, the motor had to work all the time. Further, because there is always a certain backlash in the speed of a motor, the adjustment could not be perfect.
One object of the present invention is to provide a simple, compact, relatively small, easily installable light detector roller unit which automatically corrects malfunctions in film feed in an apparatus of the type described in U.S. Pat. No. 4,312,576 which describes a device for driving the film in projectors without using the teeth of the sprocket. According to the present invention correction of film feed is accomplished by detecting the occasional malfunction in film feed by means of a light beam projected through the perforation apertures of the film and the apertures of the film support means. This is achieved by the use of fiber optic light guiding pipes between the light source and the light sensitive element so that both can be placed in a stationary position exterior of the roller unit thereby making possible the compact and simple construction of the device.
Another object of the invention is to provide a device where the film feed is essentially perfect and the correction mechanism functions only when failure in film feed occurs.